Frequently calling `auto_replenish` will reset `self.last_update` each
time and `tokens` may be a fractional value (0 since it is a `u64`), in
this case no tokens will replenished.

To avoid this we increment `self.last_update` by the minimum time
required to generate `tokens`, in the case where we have the time to
generate `1.8` tokens but only generate `x` tokens due to integer
arithmetic this will carry the time required to generate 0.8th of a
token over to the next call, such that if the next call where to
generate `2.3` tokens it would instead generate `3.1` tokens. This
minimizes dropping tokens at high frequencies.

Signed-off-by: Jonathan Woollett-Light <jcawl@amazon.co.uk>